1. Field of Invention
The present invention relates to a method of canning green vegetables which prevents to a large extent the loss of the green color as well as the natural flavor effected by heat sterilization during the canning process.
2. Prior Art
In the commercial canning of foods, the product is sealed into cans in an unsterilized condition and then heat sterilized in a retort or pressure cooker. It is characteristic of such canning procedure that considerable overcooking of the food takes place inasmuch as the sealed cans must be heated for a considerable period of time to completely kill spoilage microorganisms such as bacteria and spores present in the food.
Modern food processors use temperatures well above 212.degree. F., e.g., 240.degree. F., and long periods of heating (as high as an hour or more) to ensure adequate destruction of spoilage microorganisms. The United States National Association of Food Processors (NAFP) has developed shelf-stable sterility temperature/time parameters based on subjecting a canned food product to elevated temperatures for inverse periods of time calculated to adequately destroy spoilage microorganisms therein to a commercially acceptable and nutritionally safe sterilization level. These temperature/time parameters are identified by the term F.sub.o value (sterility value) which is basically a time equivalent calculated at 250.degree. F. Particular F.sub.o values required to achieve commercially acceptable shelf-stable sterility are highly variable, depending upon type and size of container, type and size of food product, acidity of product, and the like. Reference is directed to the National Canners Association "Laboratory Manual for the Canning Industry", Second Edition, 1956 for further information on this matter and how F.sub.o values are derived by those skilled in the art.
The NAFP recommends that vegetables such as green peas be sterilized under conditions calculated to provide an F.sub.o value of at least 6. The higher the F.sub.o value, the greater the degree of sterility. The present practice in the green pea canning art is to heat the canned peas in a No. 2 or No. 10 can at 250.degree. F. for 15-30 minutes to achieve an F.sub.o of 6 to 8. Green vegetables and particularly green peas undergo a substantial loss of organoleptic quality when sterilized under these sterilization conditions, i.e., serious impairment of the flavor and color of the product results, e.g., the sterilized green peas acquire a brownish color and a taste quite unnatural to cooked fresh peas.
To meet the problem of overcooking to effect sterilization, canning methods have been developed, e.g., U.S. Pat. No. 3,892,058, wherein the packaged food product is rapidly heated to a relatively high temperature and then rapidly cooled. This High Temperature-Short Time (HTST) procedure very markedly reduces cooking inasmuch as sterilization can be effected very rapidly. Thus, in cases where one hour of heating is required at 240.degree. F., for example, to kill all spoilage microorganisms, the same result is accomplished in less than 5 minutes at 270.degree. F.
Although HTST sterilization has the above-mentioned advantage, commercial usage of the technique has heretofore been limited largely to liquid products such as milk because of the uneven heat distribution believed to occur in solid and semi-solid foods.
The utilization of HTST sterilization for canned solid foods such as green vegetables would be particularly advantageous as it is essential that the contents of the can be thoroughly sterilized after sealing to ensure the destruction of spoilage microorganisms.
The impairment in the organoleptic qualities of canned green vegetables has been determined to be due to the development of acid caused by the hydrolytic and oxidative changes which accompany the sterilization of the green vegetables in commercial canning. Thus the normal pH value for the fluids expressed from raw peas is about 6.0 to 7.0 and in special cases may be as high as 6.9. The canned product resulting from the sterilization of canned peas is found to have a pH value ranging from 5.8 to 6.4 and most frequently from 6.1 to 6.2. This decrease in pH constitutes an increase in acidity which causes the destruction of the natural green pigment during processing and subsequent storage, i.e., the degradation of green colored chlorophyll to brown colored pheophytin and pyrole.
Attempts have been made to correct this acidity by the addition of a neutralizing agent to the brine in which the vegetable is packed. For example, it has long been known to the art, e.g., U.S. Pat. Nos. 2,473,747 and 2,989,403, that the color of green vegetables can be maintained during canning and sterilization if sufficient alkali is added to the brine to maintain a pH of 8.0-8.5. Such attempts by the prior art have not been totally successful as the neutralizing agent, having been added to the vegetable brine, prior to or at the time of canning, is substantially dissipated before the vegetables are completely cooked. Thereafter, during storage, as the organic constituents of the vegetables and the brine, e.g., sugars, are slowly oxidized by the air in the can headspace, ester and amino acid compounds present in the vegetables are hydrolyzed, organic acids form which lower the pH of the brine to values at which continued discoloration of the vegetables can occur.
As an alternative to incorporating a neutralizing agent in the brine, the art, e.g., U.S. Pat. No. 2,875,071 issued Feb. 24, 1959 to George J. Malecki, has suggested incorporating into the interior can coating, an innocuous alkaline compound which is soluble in the canning brine, the coating and the compound being of such a character that the alkaline compound will leach out of the coating into the brine over an extended time period to maintain the pH of the brine at an alkaline level, e.g., pH 8.0-8.5.
The art, e.g., U.S. Pat. No. 4,104,410, issued Aug. 1, 1978, also to George J. Malecki, further discloses that although the use of alkaline compounds such as Mg(OH).sub.2 and Ca(OH).sub.2 are effective for the retention of green color in canned peas, the pH and the intensity of the retained green color decreases gradually when canned vegetables are kept in storage, so that to achieve a satisfactory green color retention at the end of one year of storage, it is necessary to maintain the pH at such a high alkaline level that an ammoniacal (barnyard) flavor develops. The off-flavor is believed due to the hydrolysis of amides such as glutamine or aspargine and fat (lipids) present in pea tissue.
U.S. Pat. No. 4,104,410 discloses a canning method for green color retention in canned green vegetables without the use of chemical additives in the container coating or brine wherein the vegetables are maintained in an aerobic environment up to the time of sterilization, the conventional blanching operation being replaced by a relatively low temperature (e.g., 100.degree. F.) washing step. Sterilization of the canned food product is accomplished using HTST conditions of 250.degree. F. and preferably above 275.degree. F. at time periods ranging from a minute or a few seconds at high temperatures, e.g., 275.degree.-425.degree. F. to 5 minutes at 260.degree.-270.degree. F.